1. Field of the Invention
The present invention relates in general to two-way optical fiber communication systems where data can be exchanged between a central location and at least one remote location. More particularly, the present invention relates to two-way optical fiber communication systems that use only a single laser light source to establish the two-way communications between the central location and at least one remote location.
2. Description of the Prior Art
There are many applications that utilize an optical fiber network to establish two-way optical communications between a host digital terminal (HDT) at a central location and an optical network unit (ONU) at a remote location. Typically, communications from the HDT to the ONU are considered downstream communications, while communications from the ONU to the HDT are considered upstream communications. An example of such a two way optical communication would be a television cable supplier that offers a pay-per-view movie service via an optical fiber network. The cable supplier may list the movies available over the optical fiber network. The customers, via their cable boxes, may then interact with the cable supplier over the optical fiber network to select a particular movie.
At present, two-way optical communications between two locations requires that each location use its own light source for generating the optical carrier that carries the information to be transmitted. Light sources are expensive, making such optical communication systems economically impractical for many applications.
N. J. Frigo et al. have proposed in a paper entitled "RITE-Net: A Passive Optical Network Architecture Based On The Remote Interrogation of Terminal Equipment", OFC'94, San Jose, Calif., Paper No. KD8, pp.43-45, and in a paper entitled "Wavelength-Division Multiplexed Passive Optical Network with Cost-Shared Components", IEEE Photonic Technology Letters, Vol. 6, pp. 1365-1367, 1994, an optically-passive access system. The disclosure of these papers has the HDT at a central location connected to a remote location by two optical fibers. However, in the disclosed systems only the HDT has a light source, which it uses to transmit a signal to a remote location on one of the fibers. The remote location reflects some of the light it receives to the second fiber, whereby the second fiber carries the second signal back to the HDT. The modulation used in both directions of transmission is identical, in that the absence of light stands for one of the two binary symbols used (1 or 0) and the presence of light indicates the second binary symbol (0 or 1). Because of the identical modulation, reflections result in significant interference, thereby imposing constraints on system design.
Nevertheless, such prior art systems are attractive in that they have the potential of reducing costs and can be used in different system configurations that allow sharing of some of the fiber between the HDT and the remote locations.
One way of reducing system costs is to use a single fiber for both directions of transmission. This implies a need for a transmission plan in which upstream reception is insensitive to reflections of the downstream signal by using different modulation schemes in the two directions of transmission.